Pulse power electrical systems are well known. One such system is described in U.S. Pat. No. 5,142,166 issued to Birx. That patent described a magnetic pulse compression circuit which shortens and amplifies an electrical pulse resulting from the discharge of a charge storing capacitor bank. The patent also describes a induction transformer for amplifying the pulse voltage.
Another pulse power circuit is described in U.S. Pat. No. 5,729,562 issued to Birx, et al. which describes a pulse power system for a gas discharge laser and includes an energy recovery circuit for recovering electrical energy reflected from the laser electrodes.
Gas discharge lasers are well known and many of these lasers utilize pulse power systems such as those described in the two above-referenced patents to provide short high-voltage electrical pulses across the electrodes of the lasers. One such gas discharge laser is described in U.S. Pat. No. 4,959,840.
Lasers similar to the one described in U.S. Pat. No. 4,959,840 utilizing pulse power systems like the one described in U.S. Pat. No. 5,729,562 are utilized as light sources for integrated circuit lithography. At the present time, most of these lasers are configured to operate as KrF lasers utilizing a laser gas comprised of about 0.1 percent fluorine, about 1.0 percent krypton and the rest neon. These lasers produce light at a wavelength of about 248 nm.
There is a need for lithography light sources at wavelengths shorter than 248 nm such as that produced when the lasers are configured to operate as ArF or F2 gas discharge lasers which produce laser beams with wavelengths of about 193 nm and about 157 nm, respectively. In the case of the ArF laser the gas mixture is substantially argon, fluorine and neon and in the case of the F2 laser the gas mixture is substantially F2 and He or F2 and neon.
Fused silica is the primary refractive optical material used in integrated circuit lithography devices. At wavelengths in the range of 193 nm and 157 nm fused silica is damaged by high intensity ultraviolet radiation. The damage is caused primarily by double photon excitation so that for a given pulse energy, the extent of the damage is determined largely by the shape and duration of the pulse.
An electrical drawing of a prior art modular pulse power system is shown in FIG. 1. In a prior art system the components of the pulse power system are provided in a power supply module, a commutator module and a unit called the compression head which is mounted on the laser chamber.
High Voltage Power Supply Module
High voltage power supply module 20 comprises a 300-volt rectifier 22 for converting 208-volt three phase plant power from source 10 to 300-volt DC. Inverter 24 converts the output of rectifier 22 to high frequency 300 volt pulses in the range 1000 kHz to 2000 kHz. The frequency and the on period of inverter 24 are controlled by a HV power supply control board (not shown) in order to provide course regulation of the ultimate output pulse energy of the system. The output of inverter 24 is stepped up to about 1200 volts in step-up transformer 26. The output of transformer 26 is converted to 1200 volts DC by rectifier 28 which includes a standard bridge rectifier circuit 30 and a filter capacitor 32. DC electrical energy from circuit 30 charges 8.1 xcexcF Co charging capacitor 42 in commutator module 40 as directed by the HV power supply control board which controls the operation of inverter 24. Set points within HV power supply control board are set by a laser system control board in a feedback system in order to provide desired laser pulse energy and dose energy (i.e., the total energy in a burst of pulses) control.
The electrical circuits in commutator 40 and compression head 60 merely serve to utilize the electrical energy stored on charging capacitor 42 by power supply module 20 to form at the rate of (for example) 2,000 times per second electrical pulses, to amplify the pulse voltage and to compress in time the duration of each pulse. As an example of this control, the power supply may be directed to charge charging capacitor 42 to precisely 700 volts which during the charging cycle is isolated from the down stream circuits by solid state switch 46. The electrical circuits in commutator 40 and compression head 60 will upon the closure of switch 46 very quickly and automatically convert the electrical energy stored on capacitor 42 into the precise electrical discharge pulse across electrodes 83 and 84 needed to provide the next laser pulse at the precise energy needed as determined by a computer processor in the laser system.
Commutator Module
Commutator module 40 comprises Co charging capacitor 42, which in this embodiment is a bank of capacitors connected in parallel to provide a total capacitance of 8.1 xcexcF. Voltage divider 44 provides a feedback voltage signal to the RV power supply control board 21 which is used by control board 21 to limit the charging of capacitor 42 to the voltage (called the xe2x80x9ccontrol voltagexe2x80x9d) which when formed into an electrical pulse and compressed and amplified in commutator 40 and further compressed in compression head 60 will produce the desired discharge voltage on peaking capacitor 82 and across electrodes 83 and 84.
In this embodiment (designed to provide electrical pulses in the range of about 3 Joules and 16,000 volts at a pulse rate of 1000 Hz to 2000 Hz, about 100 microseconds are required for power supply 20 to charge the charging capacitor 42 to 800 volts. Therefore, charging capacitor 42 is fully charged and stable at the desired voltage when a signal from commutator control board 41 closes solid state switch 44 which initiates the very fast step of converting the 3 Joules of electrical energy stored on charging capacitor Co into a 16,000 volt discharge across electrodes 83 and 84. For this embodiment, solid state switch 46 is a IGBT switch, although other switch technologies such as SCRS, GTOs, MCTs, etc. could also be used. A 600 nH charging inductor 48 is in series with solid state switch 46 to temporarily limit the current through switch 46 while it closes to discharge the Co charging capacitor 42.
The first stage of high voltage pulse power production is the pulse generation stage. To generate the pulse the charge on charging capacitor 42 is switched onto C1 8.5 xcexcF capacitor 52 in about 5 xcexcs by closing IGBT switch 46.
A saturable inductor 54 initially holds off the voltage stored on capacitor 52 and then becomes saturated allowing the transfer of charge from capacitor 52 through 1:23 step up pulse transformer 56 to Cpxe2x88x921 capacitor 62 in a transfer time period of about 550 ns for a first stage of compression.
Pulse transformer 50 is similar to the pulse transformer described in U.S. Pat. Nos. 5,448,580 and 5,313,481; however, this prior art embodiment has only a single turn in the secondary and 23 separate primary windings to provide 1to 23 amplification. Pulse transformer 50 is extremely efficient transforming a 700 volt 17,500 ampere 550 ns pulse rate into a 16,100 volt, 760 ampere 550 ns pulse which is stored very temporarily on Cpxe2x88x921 capacitor bank 62 in compression head module 60.
Compression Head Module
Compression head module 60 further compresses the pulse.
An Lpxe2x88x921 saturable inductor 64 (with about 125 nH saturated inductance) holds off the voltage on 16.5 nF Cpxe2x88x921 capacitor bank 62 for approximately 550 ns then allows the charge on Cpxe2x88x921 to flow (in about 100 ns) onto 16.5 nF Cp peaking capacitor 82 located on the top of laser chamber 80 and is electrically connected in parallel with electrodes 83 and 84 and preionizer 56A. This transformation of a 550 ns long pulse into a 100 ns long pulse to charge Cp peaking capacitor 82 makes up the second and last stage of compression.
Laser Chamber
About 100 ns after the charge begins flowing onto peaking capacitor 82 mounted on top of and as a part of the laser chamber module 80, the voltage on peaking capacitor 82 has reached about 14,000 volts and discharge between the electrodes begins. The discharge lasts about 50 ns during which time lasing occurs within the optical resonance chamber of the excimer laser. The optical resonance chamber described is defined by a line narrowing package comprised in this example of a 3-prism beam expander, a tuning mirror and an echelle grating and an output coupler.
A typical pulse shape (power vs. time) of a prior art ArF laser with a pulse power system as shown in FIG. 1 is shown in FIG. 2. In this example the power rises fast from zero to a first peak in about 5 ns, fluctuates for about 20 ns then decreases to almost zero in about 10 ns. The total energy in the pulse is about 10 mJ. In order to minimize two-photon reactions, without reducing pulse energy, peaks should be reduced and the duration of the pulse should be lengthened. A parameter which is utilized in the lithography industry to evaluate the potential of these pulses to cause optical damage is called the xe2x80x9cintegral square pulse durationxe2x80x9d and its symbol is Tis. Tis is defined as follows:       T    is    =            ∫                        (                                    P              ⁡                              (                t                )                                      ⁢                          ⅆ              t                                )                2                    ∫                                    P            2                    ⁡                      (            t            )                          ⁢                  ⅆ          t                    
where P=power
Tis for the pulse shown in FIG. 2 is about 35 ns.
The optical pulse duration of the laser is determined by the discharge current duration and by the discharge stability time, which is a function of the fluorine concentration. The current durations of a standard laser is given by:
xcfx84=xcfx80xe2x80xa2{square root over (LC)}
L laser head inductance, C peaking capacitance
Any change in L or C is not very effective since pulse duration only increases to the square root. In the interest of laser efficiency, the laser head inductance L cannot be increased. Increasing the capacitance C slows down the voltage risetime and significantly increases the amount of energy deposited into the discharge. Both measures deteriorate the discharge quality and efficiency. Therefore a doubling of the pulse duration will not be possible with a simple LC circuit.
Long pulse duration excimers lasers have been built using so-called spiker-sustainer excitation. In this scheme the tasks of reaching gas break-down and sustaining a stable discharge have been divided into two separate systems. Gas break down requires high voltages but only low energies, which is handled by a spiker circuit. A sustainer circuit is matched to the much lower steady state discharge voltage and provides the pumping of the laser. Because the voltage of the sustainer is much lower, a larger capacitance can be used and much longer pulse duration are achievable. In XeCl laser pulse durations up to 1.5 ns have been realized using spiker-sustainer excitation.
Unfortunately, spiker-sustainer circuits are not applicable to lithography lasers. KrF and ArF discharges use F2 as the halogen donor, which makes discharges inherently more unstable and limits the pulse duration with respect to chlorine based lasers. More importantly, spiker-sustainer excitation provides low gain due to the stretched out power deposition. Lithography lasers require line-narrowing provisions that typically introduce high cavity losses. In such a configuration, the low gain can barely overcome the losses and low laser performance results.
What is needed is a pulse power system for lithography lasers which will provide a substantial increase in Tis from about 30-35 ns to about 50-60 ns.
The present invention provides a long pulse pulse power system for gas discharge lasers. The system includes a sustainer capacitor for accepting a charge from a high voltage pulse power source. A peaking capacitor with a capacitance value of less than half the sustainer capacitance provides the high voltage for the laser discharge.